We've done an experiment regarding water electrolysis. Basically, we just separated the oxygen and hydrogen gas in a beaker that contains a magnesium sulfate solution. We set the power supply voltage to 12.46V with an initial .09A and final .01A over 45 mins. I was wondering as to what causes the drop in current draw. I have my suspicion that the concentration of the solution is decreased since we're using copper wires, and there was a displacement reaction between copper and magnesium sulfate.

1 Answer
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If you look at the copper wires after the electrolysis, the cathode (connected to the negative pole of the current source) will be white. This electrode is coated with Mg(OH)2.

The low activity of copper on the electrochemical scale makes it unlikely (impossible) for copper to displace magnesium from an aqueous salt.

However, the initial voltage is high enough to theoretically evolve metallic magnesium, which would instantly react with water to produce H2 and Mg(OH)2, which will precipitate because of its low solubility. The Mg(OH)2 adheres, at least to some degree, on the copper electrode and interferes with current flow. The resistance to the electrolysis at the same voltage leads to a lower resulting current.

Even at electrode potentials below that needed to evolve metallic Mg, current carried by the aquo Mg++ ion will result in an increased pH and deposition of Mg(OH)2 around the cathode. An interesting question is whether the deposit from a low voltage electrolysis has the same adhesion as from a high voltage experiment. Maybe low voltage electrolysis would deposit Mg(OH)2 farther out in the solution and not be so blocked off by adhesion to the electrode.

$\begingroup$Along with my observed result of the electrolysis process, there was a lump of white stuff which was as you've mentioned was precipitate and how about the bluish stuff? The copper electrode we used might have had caused that. But, I was wondering as to how it has been stripped off in a solid form and adhere with assuming sulfate from the magnesium sulfate?$\endgroup$
– MIGGYEENov 24 '18 at 13:15

$\begingroup$The copper anode is subjected to acidification and oxygenation. This would tend to favor some dissolution of copper, but not stoichiometrically - i.e., not one Cu into solution for each Mg out. The white lump was Mg(OH)2 that started adhering to the cathode, then got blasted off by bubbles of H2 forming under it. Continued electrolysis is going to be a battle between H2 formation and Mg(OH)2 adhesion and blockage of the cathode surface. A better anode would be stainless steel (or Pt).$\endgroup$
– James GaidisNov 24 '18 at 14:11